The present invention relates to a lock-up control system for a torque converter of an automatic transmission for a vehicle.
Automatic transmissions including or equipped with a torque converter have a disadvantage that since there is a slip within the torque converter between the pump impeller and turbine runner even when the vehicle runs at high speeds when the torque duplication by the torque converter is not required, a loss in power transmission under this vehicle operating condition is noticeable, resulting in the poor fuel economy of the vehicle.
In order to improve the fuel economy of the vehicles having an automatic transmission of the above mentioned type, it is known to provide a lock-up mechanism, such as a direct drive clutch, within the torque converter so as to selectively engage to provide a direct drive connection between the pump impeller and turbine runner for the purpose of preventing the occurrence of the slip.
The conventional lock-up control systems are such that the lock-up mechanism is shifted into the engaged condition to provide the direct drive connection between the pump impeller and turbine runner upon a shift is made into a predetermined speed in the automatic transmission.
A problem with the conventional lock-up control system is that the torque converter may lock up before the shifting operation into the predetermined speed has been completed, resulting in occurrence of a great shock and thus the riding feeling is degraded or becomes uncomfortable and the abnormal vibration due to such shock may cause a great local stress on the associated mechanical parts, shortening the torque converter life.
The present invention has been contemplated to solve the above problem.